The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Both linear voltage regulators and DC/DC converters have been used to supply regulated power to circuits of a device. Linear voltage regulators typically include a transistor that drops an input voltage to a regulated output voltage. DC/DC converters typically include one or more diodes, switches, capacitances and/or inductances that store and release power. DC/DC converters can provide regulated output voltage both above and below the input voltage.
Low-dropout (LDO) regulators are one type of linear voltage regulator. Dropout refers to a minimum difference between input and output voltage that sustains regulation. Although the efficiency of the LDO regulator is generally lower than the DC/DC converter, it may be offset by the relatively low cost of the LDO regulator.
When systems employ both LDO regulators and DC/DC converters to supply power to the circuits of the device, there can be situations when the LDO regulator will experience voltage droop. The droop in voltage may fall below a voltage or power level floor required for the circuit.
FIG. 1A is a diagram showing ideal output voltage as a function of time based on output of a DC/DC converter and a LDO regulator that are used to drive a circuit. During an active or high power (HP) mode, the DC/DC converter supplies power to a target level as shown at 80. During a standby or low power (LP) mode, the DC/DC converter is generally or substantially turned off. During this period, the LDO regulator takes over and ensures that a minimum average voltage (or power) is maintained at a minimum level as identified by 82. The minimum average power may allow the modules of a driven circuit to maintain states and/or to reduce start-up delay that may otherwise occur if power was not supplied during the standby mode.
Consequently, the DC/DC converter supplies power during the active mode while the LDO regulator maintains the floor during the standby mode. During a period t0 prior to transitioning to the active mode, the driven circuit may start using a little more power to initiate turning-on one or more of the other modules of the circuit so that they can be ready to operate in the active mode. During the period t0, the LDO regulator is operating to ensure that minimum power or the floor 82 is supplied to the circuit. As a result of the increase in power supplied to the circuit, Vout may droop below the floor 82 to be maintained by the LDO regulator.
FIG. 1B is a diagram showing a typical output voltage Vout as a function of time based on the output of the DC/DC converter and the LDO regulator. The voltage is maintained at the floor 82 as shown at 84 by the LDO regulator. Then, the voltage droops below the floor 82 as shown at 86 during the period t0. Then, the voltage increases at 88 due to the output of the DC/DC converter increasing and supplying an active voltage level. Then, the voltage falls after the active mode ends and the DC/DC converter is off. A rate or time constant of the falloff may be based on values of components of the impedance (and possibly other circuit impedances) and a rate of power consumption by the driven circuit.